Let's continue with our discussion of VLAN Trunking. We're going to look at both the ISL and the 802.1Q packets as they come across a Trunk Port. So first we'll look at the ISL encapsulated packet and I kind of liken them to the Russian nesting dolls that you see down here. Basically ISL takes an untagged and unencapsulated Layer 2 Frame from an Access Port and just slaps another header and another footer around it. So it basically takes this doll here and puts it inside that one and sends that across the port. The other end looks at this packet says, oh it's an ISL Frame, I'm going to take off this ISL header and footer, oh look there's an Ethernet packet inside there. We'll, this ISL header said in it's VLAN section that it should be VLAN 248, so whatever ports I have for VLAN 248, I'm going to send this untagged, unencapsulated packet out those ports. Sounds like a whole lot of work doesn't it? Well it, it was, however at the time, it was kind of state of the art. This way you don't have broadcasts running amok all over your network. You only have broadcasts for specific VLANs crossing those Trunk Ports. Now really the only thing you have to worry about in the ISL header is that VLAN Tag that you see there right in the middle. There's all kinds of other fields in the ISL header and basically that's because Cisco intended ISL to be the be all and end all when it came to VLAN Trunking. So they wanted to be able to support all kinds of different protocols and all kinds of different bits and bobs as it were inside ISL. Unfortunately, the IEEE chose 802.1Q and so that's why ISL is being phased out. So in reality you'll probably never see an ISL Trunk on a modern production network but you do have to know this for the exam. You have to know that ISL encapsulates and it has the VLAN ID in the ISL header. So let's look at the 802.1Q Frame. Now as you see here, we have a plain old, IEEE 802.3 Ethernet Frame. It's got all of the information in it and instead of encapsulating, what 802.1Q does, is right after the source address and right before the data length, it inserts a little shim if you will, right, right there. And says here's the VLAN priority, here's the VLAN identifier, it recalculates the Frame Check Sequence, sends it out the Trunk Port. A lot simpler, a lot less overhead on all the switches involved and more importantly it's supported by every switch manufacturer in the world. So again this is what you'll see in most production networks. It's a lot cleaner, you can actually look at the packet in Wireshark and you can actually see the source and destination address without having to actually decode that ISL header and footer. You can just look right at the Ethernet packet and in fact, Wireshark actually sees a Tagged Ethernet packet as just a plain Ethernet packet. It just happens to have VLAN information embedded in it. So let's move onto one of the most common errors that I see when you're dealing VLAN Trunks and that's the error right there. Native VLAN Mismatch and every Cisco engineer that's reading this for review purposes just threatened to punch their laptop screen. Native VLAN Mismatch is not a show stopper, it's not a huge problem in most situations. Now you might ask what is a Native VLAN? This is the first I've heard of a Native VLAN. Well a Native VLAN is used for untagged packets that are received on a Trunk Port and you might be thinking, well if it's a Trunk Port, then it's connected to another switch and there should only be two devices on that link. So why do I need a Native VLAN? Well there's been two situations that I have seen where Native VLANs come in handy. Number one is there's a hub between the Trunk Ports so you have a set up that looks kind of like that and that little funky device right here in the middle is a hub and you've got machines connected to this hub. Well obviously this hub does not do any kind of VLAN Tagging so it's just sending these Ethernet packets out to both of these switches, completely untagged and in the clear. And so what these switches do, is say well I'm receiving an untagged packet on this port, I'm going to dump into VLAN 10. Now let's say on this end, the Native VLAN is 10, the Native VLAN on this end is 20, well obviously you'll get inconsistent communication, because you're communicating with different machines on this end of the link as you are on this end of the link. Also you could potentially leak traffic from VLAN 10 on this side to VLAN 20 on this side and so that's why you'll end up with this error message. It's saying hey the other end has a different Native VLAN than I do, you need to correct it, if it's truly an issue. Now in reality, as an Enterprise Network Engineer, if you come across a hub in your topology, you're just inviting disaster and I recommend that you kill it with fire as quickly as possible, nuke it from orbit, it's the only way to be sure. However, it does happen, you'll have a hub that's out in a remote building someplace, that happens to have a manual switch on the other end and it's not optimal but especially in a lot of older networks or smaller networks even, you'll run into something like this. The other situation where I've seen it is for VOIP and in this case, we'll replace all of this with these two devices. You have Voice Over IP phone that has a port that connects to the switch and a port that connects to the PC sitting on the persons desk so that way you only have to have one Data Drop from the switch to the cubicle or the office or wherever. Now there are situations where you would not tell the IP phone to tag the packets coming in from this PC. So it passes the packets from this PC along completely untagged to this switch. This switch would say, hey well I'm going to dump these packets into Native VLAN. Now that's the old way of doing it, the new way of doing it is to actually set up a Voice and an Access VLAN on this Switch Port here and then there's magic CDP that happens between these two devices that says, hey I'm a phone and anything else that's not voice traffic, you will tag it and dump it on whatever VLAN you want. So a Native VLAN is not really needed in modern deployments but again, this used to be the Cisco approved way of doing it and you will still see it on some networks as you make your travels through the world. And obviously how to set all that up is well beyond the scope of a switch course but this is the other valid reason for having a Native VLAN. And that concludes our discussion of VLAN Trunking.
|Course:||Implementing Cisco IP Switched Networks (642-813 SWITCH)|
|Duration:||8.5 hrs / 102 lessons|
|Captions:||Available on CD and Online University|
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